1. Field of the Invention
This invention relates to a transistor, particularly to the structural configuration of a high power bipolar transistor.
2. Description of Prior Art
A bipolar transistor comprises a collector region having a first conductivity, a base region having the opposite conductivity and formed in the collector region and at least one emitter region formed in the base region, with electrodes being arranged on their respective region surfaces.
In such a transistor, the emitter region is generally formed in the base region in the form of island at a uniform depth and the electrode is formed and connected at almost the center of the surface.
When the transistor having such configuration is in ON state, wherein the forward bias is being applied between emitter and base, carriers are injected from the base to the emitter and are dispersed to the adjacent areas of the emitter region. However, when zero bias or back bias is applied to the emitter-base junction, (namely, the transistor biased in its OFF state), residual carriers are generated under the emitter region and by this influence, current is concentrated almost at the center of said emitter region and secondary breakdown results.
In general, in the case of power transistors, called overlay type transistors, wherein a plurality of emitter regions are formed in one base region and said emitter regions are connected in parallel by means of the metal electrode, the above-mentioned problems occur in each emitter region.
For this reason, it was very difficult to form such a high power transistor with the existing emitter configuration.
Moreover, in U.S. Pat. No. 3,427,511, a configuration of a transistor is shown, wherein the active emitter region is formed in the base region in the form of a ring, thus reducing the aforementioned incidence of secondary breakdown. In the configuration shown in this patent, the higher resistivity region is formed at the entire surface of the base region surrounded by the ring type active emitter region, and to which the emitter electrode is arranged and connected.
Even in this configuration, resistance of the base region under the emitter region is high due to the existence of a higher resistivity region on the entire surface, and therefore residual carriers are generated, thereby making the occurrence of secondary breakdown likely.